Synesthesia is a harmless perceptual condition is which the stimulation of one sense triggers experiences in another sense. For example, in a synesthete, hearing music might cause a synesthete to experience colors or textures. Or a sound might trigger a taste, or the concept of a weekday may trigger a color.

So there are many forms of synesthesia?

There are many possible cross-pairings or senses and concepts, probably over one hundred.

What percentage of the population has synesthesia?

It used to be thought to be quite rare, but we now know that about 4% of the population possesses some form.

Is there a term for someone with synesthesia?

A synesthete.

Can one person possess many forms of synesthesia?

Yes. In fact, if you have one form, such as colored letters, then you're quite likely to have another form, such as colored months.

Are some types more common?

Some types are much more common than others. For example, the most common experience seems to be that sequences (such as weekdays or numbers) trigger a sense of spatial location (as in, 'Sunday is just beyond my left shoulder'). Second to that, sequences seem to trigger colors. Other types, such as a sound triggering a smell, is much more rare.

Is it random which types someone might have?

My lab has just discovered that the types tend to cluster. If you have colored letters, you're likely to have colored months, weekdays or numbers -- but you're no more likely than anyone else in the population to have, say, hearing-to-taste synesthesia.

What does that clustering mean?

We hypothesize that there may be several different conditions--and perhaps several different genetic bases--underlying the different clusters of synesthesia.

How do you know if someone is faking it? Or just claiming to have this for attention?

My lab and others have developed rigorous tests that only synesthetes can pass. Essentially these tests pivot on consistency: a letter-color synesthete will always pick the same color for a letter every time you present it, even years later. Someone faking it can't pass that test.

Is there a place where people can take the tests to determine if they have synesthesia?

No, it's not a hallucination, but an internal experience. For a synesthete, the number 3, say, triggers an internal experience of, say, purple. It's just self-evidently true to the synesthete that 3 and purple are equivalent.

Do all synesthetes experience the same colors for the same letters?

No. Their experiences are totally idiosyncratic. One synesthete's experiences is not like another's.

How many synesthetes have you studied in your laboratory?

We now have over 6,000 tested, verified synesthetes.

Would you say that they actually experience a different reality?

That's exactly right. There's an old question in philosophy about whether what your internal experience of the color we call Ã¢â‚¬Å“blueÃ¢â‚¬Â is the same as my experience of blue. Maybe I see blue the way you see red. It's an old question, but it turns out that the truth may be run even deeper. With synesthesia we can study and quantify the way that someone's reality can be different than someone else's. It's moved from philosophy to the laboratory.

Why do you study synesthesia?

Aside from the ability to study a different reality, it also serves as a powerful inroad into how the normal (non-synesthetic) brain works. The brain takes in all the senses through different channels, and constructs a unified reality from that. How it does this is an unsolved question in neuroscience, known as the binding problem. Synesthesia allows us to see how different brains do this differently.